Firstly, image-based tabs will only be displayed when the <tt>#TAB_IMAGE#</tt> substitution string is used in the tab attributes in the page template (see the "Two Level Tabs - Right Sidebar (optional / table-based)" template in your app where I have made this modification).

Secondly, image-based tabs are subject to a long-unfixed bug where the image source always uses the <tt>#IMAGE_PREFIX#</tt> substitution string (see +{thread:id=2132745}+ among others) which has to be worked round if your images are not in the virtual image directory. One way to do this is using a Dynamic Action to strip out the unwanted <tt>/i/</tt> from the URL path:
h4. When

Event: Page Load

h4. True Action

Action: Execute JavaScript Code
Code:

$('.tab-holder img') // Note that selector for tab images is theme-dependent
.attr('src', function(i, src) {
return src.slice(3) // Slice index depends on #IMAGE_PREFIX#: this works when it is "/i/"
});

However&mdash;even if this bug is ignored&mdash;using image-based tabs in 2013 is a backward step. Personally I avoid using tabs at all: lists are much more flexible, and easier to implement.